Tougher Beans

Steve Henry, who farms near Nevada, IA, is trying to learn everything he can about Sudden Death Syndrome (SDS). However, he's frustrated with the lack of hard data about the disease.

He began noticing yield loss from SDS in 2003. That year, a 50-acre field with the disease averaged 32 bu./acre while his whole farm average was 52 bu./acre. “When SDS hits, it can be very devastating,” he says. “And I've noticed that some soybean varieties are less tolerant to the disease than others.”

Henry has learned a few things to help manage SDS, but he'd like more information. “There's not a lot of data available to help us accurately manage this. We need to figure it out,” he says. “We aren't even sure which varieties are more tolerant to SDS. The seed companies have no standardized way to rate SDS tolerance. We're lacking good information.

“I don't want to discount the impact or seriousness of Asian soybean rust, but we have diseases that are robbing our yields right now,” Henry says. “I think more research dollars should be spent toward formulating effective management plans to combat the diseases we have right here in Iowa.”

SDS and soybean cyst nematode (SCN) account for as much as 60-70% of total yield loss in Iowa alone — close to 70 million bushels, according to Palle Pedersen, soybean Extension agronomist at Iowa State University. “SDS and SCN have been confirmed in more than 75% of Iowa fields,” he says. “Growers will never be able to make money on soybeans if they don't manage the soybean plant as if they could make money on it.”

Pedersen says SDS is caused by a pathogen called Fusarium solani, which can be extracted out of almost every field in North America. But certain environmental conditions need to be in place for the disease to be expressed. SCN “opens the gate” for SDS.

Terry Niblack, an Extension SCN specialist at the University of Illinois, explains that generally SDS is expressed when soybeans are exposed to a cool, wet spring, which allows the fungus to grow. The disease is often triggered by hot, dry conditions in July or August. “This isn't always the case,” she explains. “It seems to be the pattern, but it's hard to prove in an experiment.”

Niblack says SDS may or may not affect yield. Potential yield loss depends greatly on the timing of infection and weather conditions during the growing season, and if SCN is present.

Farmers may be able to identify the disease by splitting the plant's stem near the first couple of nodes, she says. While another disease, brown stem rot, will cause the pith to be brown, SDS causes the tissue surrounding the pith to be discolored. Farmers can confirm their findings by sending samples to a university plant diagnostic clinic to confirm whether or not the disease is present in that particular field.

“We think the SDS yield loss moves with the nematodes, so if you get high nematode pressure, you start seeing more SDS symptoms,” Pedersen says. “SDS by itself might not cause a lot of yield loss, but the combination of SDS and SCN can cause a tremendous loss.”

This year, Pedersen says, the disease was prevalent in Illinois and the eastern two-thirds of Iowa. “There is no doubt that we have seen a lot of SDS in Iowa over the last three years,” he says.

One of the best management tools is to plant a variety that's resistant to both diseases, but Pedersen explains that variety selection isn't foolproof. “Sometimes farmers think they're doing the right thing by selecting an SCN-resistant variety. However, that variety might be susceptible to SDS. A farmer in central Iowa can't just select a good SCN resistant variety — SDS resistance is also needed and that is now available,” he says.

The researcher urges growers to use variety trial data and then do their own on-farm testing to see how the varieties will perform on their farms. Seed companies have different rating systems and it can sometimes be a challenge to gauge effectiveness. “Every farmer, whether he has 50 or 50,000 acres, should have a small experiment on his farm with replicated strips across the field. The data he collects can be used with other trial information to help choose effective varieties,” Pedersen explains.

In the past, there was some yield drag with SCN-resistant varieties, but Pedersen says that's no longer the case. In addition, the SCN-resistant seed costs the same as non-resistant varieties.

However, as farmers manage for more and more diseases, it becomes more challenging to find varieties that will be effective. Growers with iron chlorosis, for example, will find it challenging to find a variety that will take care of iron chlorosis, SDS and SCN. But fortunately, Pedersen says iron chlorosis generally affects relatively small portions of fields compared to SCN and SDS.

“Controlling the nematode will go a long way toward making it less likely that SDS will show up,” says Niblack. “If you can find a good variety that's resistant to both diseases, if you have them, that's a good choice.”

For more information on variety selection, you can visit www.vipsoybeans.org to access SDS variety ratings, or request a hard copy of the ratings from the Illinois Soybean Association (www.ilsoy.org).

“We can't control Mother Nature, but we can build a platform for healthy plants that can better withstand disease pressure,” Pedersen says. “Plant and root health is extremely critical for high soybean yields. SCN and SDS should be on all farmers' radar since they together account for more than 50% of the yield loss from pathogens in most fields. And this is not just in Iowa — SCN is the most yield limiting pathogen in most soybean growing states in the U.S.”